Knowledge of the driving resistance of a motor vehicle is of fundamental importance in enabling optimum control of the shifting behavior of an automated shift transmission, i.e., control which is adapted to the driving situation at the time. This is because the determination of the shift speed at which the currently engaged gear should be disengaged by a shift operation and that of the target gear into which the shift should be made are essentially influenced by the current driving resistance of the motor vehicle.
As is known, the driving resistance F_fw of a vehicle is the sum of the road inclination resistance F_steig, the rolling resistance F_roll and the air resistance F_luft, i.e., F_fw=F_steig+F_roll+F_luft. In accordance with the generally known driving resistance equation:F—zug =F—steig+F_roll+F—luft+F—träg=F—fw+F—träg the driving resistance is related to the traction force F_zug of the drive engine acting on the drive wheels of the motor vehicle and the mass moment of inertia F_träg of the motor vehicle, the latter being the product of the current vehicle mass m and the current acceleration a of the vehicle in accordance with the equation F_träg=m*a. The driving resistance can therefore be written as:F—fw=F—zug−m*a 
Whereas the determination of driving resistance F_fw before and after a shift, i.e., with a gear engaged and the engine clutch engaged, presents no problem and can be done in the known way by calculating the traction force F_zug from the torque of the drive engine and the transmission ratio of the gear engaged and by calculating the mass moment of inertia F_träg from the current mass m and the current acceleration a of the motor vehicle, to determine the driving resistance F_fw during a shift operation is difficult since, during this phase, forces and torques which are largely unknown and difficult to measure act upon the output side of the drive train.
Thus it is largely unknown how quickly the torque of the drive engine falls and increases again during a shift operation; what fraction of the torque is lost due to synchronization of the target gear, and what effect speed fluctuations that can be caused by the shifting process, such as by disengaging and engaging the engine clutch, disengaging the load gear and synchronizing and engaging the target gear or, due to unevenness of the road, have on the driving dynamics of the motor vehicle, i.e., on its actual acceleration. Because of these problems, previous methods for controlling a transmission shift have not included any attempt to determine the driving resistance during a shift operation.
If the driving resistance remains largely constant before, during and after a shift, this is completely unproblematic, since the intended gear selected by the shift program on the assumption of an approximately constant driving resistance F_fw is generally optimally adapted to the driving situation at the time.
But if the driving resistance F_fw changes markedly during a shift, this can lead to problems, because the shift program of the transmission control unit begins from an erroneous driving resistance F_fw and the changed, correct driving resistance F_fw can only be determined after a time delay following the completion of the shift operation.
For example, if the motor vehicle drives from level ground onto a steeply rising stretch during an upshift, then owing to the increased driving resistance F_fw, a downshift to a lower gear should take place immediately on completion of the upshift. But if this downshift takes place too late, because of a delay in determining the new, higher driving resistance F_fw, in some circumstances the motor vehicle may come to rest if the drive engine torque, modified because of the gear engaged at the time, is no longer large enough to produce a sufficiently high traction force F_zug.
This problematic situation can be avoided, apart from by an immediate downshift, also by changing the target gear during the upshift to a lower gear, such as a gear, between the load gear and the target gear (upshift to a lower gear), by remaining in the original load gear (target gear=load gear, no transmission ratio change) or by shifting to a gear lower than the load gear (the upshift becomes a downshift). Such reactions, however, require early knowledge of the changed—in the above example, increased—driving resistance F_fw.
Accordingly, the purpose of the present invention is to indicate a method for determining the driving resistance F_fw, such that a change of the driving resistance F_fw, during a shift operation, can be determined at an early stage so that if a correction of the shift is required, it can be carried out at the right time.